Lessening the Effects of Visual Illusions


An analysis of 76 approach and landing accidents and serious incidents conducted for the Flight Safety Foundation (FSF), including controlled-flight-into-terrain (CFIT) accidents, worldwide between 1984 and 1997, found that twenty-one percent involved flight crew disorientation or visual illusions, and that poor visibility was a circumstantial factor in 59 percent of the accidents and incidents.

Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 5.3 — Visual Illusions recommends that flight crews should assess approach hazards during the approach briefing and offers the following advice:

Terrain Awareness

"When requesting or accepting a visual approach, the flight crew should be aware of the surrounding terrain features and man-made obstacles."

"At night, an unlighted hillside between a lighted area and the runway may prevent the flight crew from correctly perceiving the rising terrain."

Type of Approach

"At night, whenever an instrument approach is available (particularly an instrument landing system [ILS] approach) the instrument approach should be preferred to a visual approach, to reduce the risk of accidents caused by visual illusions."

"If an ILS approach is available, fly the ILS and use VASI or PAPI for the visual portion of the approach."

"If an ILS approach is not available, a nonprecision approach supported by a VASI or PAPI should be the preferred option."

"On a nonprecision approach, do not descend below the MDA(H) before reaching the visual descent point (VDP), even if visual references have been acquired."

"To help prevent transitioning too early to visual references and descending prematurely, the pilot flying (PF) should maintain instrument references until reaching the VDP."

"During a visual or circling approach, when on the base leg, if the VASI or PAPI indicates that the aircraft is below glide path, level off or climb until the VASI or PAPI indicates on-glidepath."

Flight Path Monitoring

"Resisting the tendency to pitch down or to descend intentionally below the appropriate altitude is the greatest challenge during the visual segment of the approach. This includes:

  • Pitching down toward the approach lights in an attempt to see the runway during a precision approach; or,
  • Descending prematurely because of the incorrect perception of being too high."

"The pilot not flying (PNF) must maintain instrument references, including glideslope deviation, during the visual portion of an ILS approach."

"Monitoring the VASI or PAPI, whenever available, provides additional visual references to resist the tendency to increase or to decrease the rate of descent."

"The following can counter visual illusions (and prevent a flight crew from descending prematurely):

  • Maintain an instrument scan down to touchdown;
  • Cross-check instrument indications against outside visual references to confirm glide path;
  • Use an ILS approach whenever available;
  • Use a VASI or PAPI, if available, down to runway threshold; and,
  • Use other available tools, such as an extended runway centerline shown on the flight management system (FMS) navigation display, ILS-DME (distance measuring equipment) or VOR (very-high-frequency omnidirectional radio)-DME distance, altitude above airport elevation to confirm the glide path (based on a typical 300-feet/one-nautical-mile approach gradient)."

Further Reading

Flight Safety Foundation

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